A wireless local area network (WLAN) basically supports an access point (AP) for a distributed system (DS) and a basic service set (BSS) formed of a plurality of stations (STA).
A Medium Access Control (MAC) protocol of a WLAN operates based on a carrier sense multiple access/collision avoidance (CSMA/CA). Accordingly, a WLAN may waste resources in channel competition. In order to overcome such a problem, an enhanced MAC protocol defined in IEEE 802.11 (e) introduced a burst transmission scheme. The burst transmission scheme transmits multiple MPDUs with a short inter-frame space (SIFS) during a transmission opportunity time (TXOP) when a transmission right for a wireless resource is obtained. Further, the burst transmission scheme receives a Block ACK frame in response to the multiple MPDUs.
Further, IEEE 802.11(n) defines an Aggregated MSDU (A-MSDU) scheme and an Aggregated MPDU (A-MPDU) scheme. The Aggregated MSDU (A-MSDU) scheme packs multiple MSDUs together into an A-MSDU and transported within a single MPDU and the Aggregated MPDU (A-MPDU) scheme packs multiple MPDUs together into an A-MPDU and transported as a single transmission unit in order to reduce overhead such as SIFS intervals. In general, a minimum SIFS interval is maintained when MPDUs are individually transmitted.
Lately, users for a WLAN have been abruptly increased. In order to increase data throughput provided from one BSS, IEEE 802.11 (ac) has been developing a standard for a very high throughput (VHT) WLAN system.
The VHT WLAN system is capable of supporting a maximum throughput of 1 Gbps for MAC SAP of an access point in three multiple stations environments including one access point (AP) and two stations (STA) and supporting a maximum throughput of 500 Mbps for MAC SAP of a wireless station in point-to-point environment. Each access point (AP) and station of a VHT WLAN are designed to support comparability to an existing WLAN system (IEEE 802.11a/n).
A wireless station obtaining a transmission opportunity is required to receive response through an ACK frame or a response frame in response to a transmitted frame in order to enhance reliability of wireless communication in a wireless communication system such as WLAN.
Such response includes an immediate response and a delayed response. The immediate response is a response (ACK frame) for single data. The block response for successive transmission or an aggregated MPDU corresponds to both of the immediate response and the delayed response.
The immediate response is to transmit a response frame with a short time interval such as a short inter-frame space (SIFS) when PHY-RXEND.primitive is generated for a received request frame in order to prevent other stations from transmitting data. As described above, a request and response frame exchange sequence is performed as a pair. Here, the immediate response frame such as an ACK frame or a CTS frame may not include a transmission address.
Unlike the immediate response, the delayed response transmits an ACK frame at first as a basic response that informs whether a request has been received or not and transmits a response frame including requested information later. The response frame is transmitted through an enhanced distributed channel access (EDCA) or based on a piggyback scheme with other frame. In the delayed response, a transmitting station receiving a response frame from a receiving station informs the receiving station whether a response frame has been received or not through an ACK frame again.
As described above, the request and response frame exchange sequence in a WLAN may be adapted at a multi-user wireless communication system. In case of supporting a multi-user multiple input multiple output (MU-MIMO) technology for uplink, wireless stations receiving frames may transmit response frames at the same time after a predetermined interval (IFS) between frames according to the frame exchange sequence.
Meanwhile, a wireless station receiving a frame is required to transmit a response frame with a time interval or using different channels for better throughput regardless of supporting the MU-MIMO technology for uplink.
A method for exchanging frames with a time interval and using a same bandwidth in a WLAN system includes a method for exchanging a response request frame and a response frame through channel access by a wireless station, and a method for sequentially transmitting a response frame at a plurality of wireless stations through one request frame.
That is, in the method for exchanging a request frame and a response frame through channel access by a wireless station, an access point (AP) transmits a request frame to a first wireless station through channel access and the first wireless station transmits a response frame with a short time interval such as SIFS. Then, the access point (AP) transmits a request frame to a second wireless station through second channel access and the second wireless station transmits a response frame with a short time interval such as SIFS.
In the method for sequentially transmitting response frames at a plurality of wireless stations through one request frame, one request frame including information on addresses of multiple users is transmitted through channel access. Then, a plurality of terminals sequentially transmits response frames with a short inter-frame space in response to the request frame.
Such a related request and response frame exchange sequence is performed after deciding a transmitting station and a receiving station, and the request frame and the response frame are exchanged with a short time interval such as SIFS in order to prevent other wireless station from obtaining a transmission right.
However, there has been a demand for developing a method for exchanging a request frame and a response frame effectively although a response frame does not include information on an address of a transmitting station like an ACK frame for a data frame or like a CTS frame for a RTS frame in a MU-MIMO wireless communication environment.